In industries where workers must operate vehicles, including heavy machinery, in a high particulate environment, it is common for the vehicle to have an air filtration system either incorporated into the vehicle's heating and cooling system or free standing within the vehicle's cab. Filtration systems are essential, as extended exposure to air containing large amounts of particulate can be harmful to the worker. These filtration systems generally consist of a single filter placed to filter fresh and recirculated air within the vehicle's cab. In such systems, there is a trade off between the amount of filtering performed and the filter life. If a fine filter is used to clean the air thoroughly, it must be changed often because it fills with particulate quickly. If a coarse filter is used, it need not be changed as often, but allows a larger amount of particulate to remain in the air. This larger amount of particulate is less desirable because of the increased risk to the worker.
Some systems employ a two stage filter having a fine and a coarse filter to extend the life of the fine filter while still providing thorough filtration of the air. In a two stage filtration system, the air is routed first through a coarse filter, then through a fine filter. The fine filter is then subject to less particulate and lasts longer. This two stage filtration system is a marked improvement over the single filter systems, but both filters must still be changed often. Also, the filter life and efficiency is not maximized because the filter system utilizes both filters regardless of how much particulate is in the air being filtered.
A controlled filtration system that maximizes the life of the filters and thoroughness of the air filtration is needed. Such a system would respond to changing conditions in the cab and optimize the use of the fine and coarse filters to maximize their life and the filtration efficiency.